This invention relates to display devices.
One specific class of display devices is those that use an organic material for light emission. Light-emissive organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolino)aluminium (“Alq3”). in a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1 shows the typical cross-sectional structure of an organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 2 such as indium-tin-oxide (“ITO”). Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a second electrode 4, which is typically a metal or alloy. Other layers can be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
There are several approaches available for the processing of conjugated polymers such as PPV. One approach uses a precursor polymer which is soluble and can therefore be easily coated by standard solution-based processing techniques. Examples of coating techniques include: spin coating, blade-coating, reverse roll coating, meniscus coating, contact/transfer coating, and ink-jet printing. The precursor is then converted in situ by suitable heat treatment to give the fully conjugated and insoluble polymer. Another approach uses directly soluble conjugated polymers which do not require a subsequent conversion stage. Depending on the specific application, one or other of the approaches might be preferred. The precursor polymer approach can be especially useful where subsequent processing might lead to damage of the polymer film if it were directly soluble—such processing may be, for instance, coating with further polymer layers (for example, transport layers or emitting layers of another colour), or patterning of the top electrode. Converted precursor films also have better thermal stability, which is of importance both during fabrication and for the storage and operation of devices at high temperatures. FIG. 2 illustrates one arrangement for depositing light-emissive polymers by ink-jetting, where a glass sheet 10 is coated with an electrode 11 and light-emissive material 12 can then be deposited by ink-jetting on to the electrode 11. A second electrode can then be deposited over the light-emissive material. (See, for example PCT/WO98/24271, the contents of which are incorporated herein by reference).
When light is produced in an electroluminescent display or other light emitting device it is emitted in all directions. In a device of the type described above some light is emitted forwards, in a viewing direction, through the transparent electrode to the viewer, whilst some is emitted backwards to the opaque metallic electrode where it is either reflected forwards to the viewer or absorbed. Another portion of the light, the portion that is emitted or scattered to more oblique angles, can be waveguided within the emissive layer or within other layers such as the transparent electrode or charge transport layers. The part of the waveguided light that is not absorbed can eventually reach the edge of the emissive pixel. This light is travelling in a direction roughly normal to the principal viewing direction and will not contribute to the brightness of the device as seen by the viewer (see N. C. Greenham et al., Advanced Materials 6 (1994) p491).
The optical structure of the device, and specifically the thicknesses and refractive indices of the component layers, plays an important role in determining how efficiently it is possible for emitted light to be contained within the plane of the device, and thus move away from the electrically-driven pixel. For example, it is possible for light to be ‘trapped’ in ‘slab waveguide’ modes which propagate within the plane of a device of the type shown in FIG. 1. A general condition for waveguiding in a region of material is that the region should have a higher refractive index than the materials on either side of it. The emissive organic semiconducting layers can themselves act as this higher refractive index region, in which waveguiding can occur, since these materials commonly show higher refractive indices than the optically transparent materials, such as inorganic glasses or organic polymers which are used as substrate, cladding or insulating layers. The occurrence of this type of waveguiding has been described in some detail in the context of optically-stimulated gain in structures made with such materials, as described for example in: “Spectral Narrowing in Optically-Pumped Poly(p-phenylenevinylene) Films”, G. J. Denton, N. Tessler, M. A. Stevens and R. H. Friend., Adv. Mater. 9, 547-551 (1997), “plastic lasers: comparison of gain narrowing with a soluble semiconducting polymer in waveguides and microcavities”, M. A. Diaz Garcia, F. Hide, B. J. Schwartz, M. D. Mcgehee, M. R. Andersson and A. J. Heeger, Appl. Phys. Lett. 70, 3191-3193 (1997), and “Light amplification in organic thin films using cascade energy transfer”, M. Berggren, A. Dodabalapur, R. E. Slusher and Z. Bao, Nature 389, 466-468 (1997).
One common type of display comprises an array of light-emissive regions that can be controlled as independent pixels to allow a desired pattern to be displayed. The array is normally planar, with the light-emissive regions and their associated electrodes and other circuitry formed on a substrate such as a glass sheet In a device of this type the obliquely emitted light travels in a direction generally in the plane of the display. In a multi-pixel device the waveguided light can cause further problems by causing cross-talk between pixels and reducing the contrast between emitting and non-emitting pixels.
The present invention aims to at least partially address one or more of these problems.
According to one aspect of the present invention there is provided a display device comprising: a light-emissive structure including two regions of light-emissive material for emitting light in a viewing direction, the regions being spaced apart in a direction perpendicular to the viewing direction and the light-emissive structure being capable of guiding light emitted from one of the light-emissive regions towards the other emissive region; and a barrier structure located between the light-emissive regions for inhibiting the propagation of light guided from the said one of the light-emissive regions to the other light-emissive region.
The barrier structure may be a light-absorbent barrier structure or a light-reflective barrier structure. The barrier structure is preferably capable of redirecting in a viewing direction light emitted from the said first light-emissive region towards the barrier structure. Such light could be emitted directly towards the barrier structure or could be waveguided to the barrier structure.
The barrier structure preferably comprises an electrode for injecting electrical charge into the first light-emissive region.
The barrier structure preferably comprises an electrically insulating formation (which may be light-transmissive) and a light-reflective layer. The light-reflective layer is preferably formed over an upper surface of the insulating formation, which surface is preferably shaped to support the light-reflective layer in a configuration in which it is capable of absorbing light that reaches it from the first light-emissive region or reflecting such light out of the device in a viewing direction. The light-reflective layer may be provided by an electrode of the device, preferably the cathode.
The electrically insulating formation suitably defines wells for receiving the light-emissive material during formation of the device. The light-emissive material maybe deposited into these wells by ink-jet printing.
The light-emissive structure may comprise an electrode for injecting charge into the first light-emissive region, suitably the anode electrode. That electrode may be capable of guiding light emitted from one of the light-emissive regions towards the other emissive region.
According to a second aspect of the present invention there is provided a display device comprising: a light-emissive structure formed on a substrate and including a region of light-emissive material for emitting light in a viewing direction; and a light-reflective structure formed on the substrate alongside the light-emissive structure for redirecting in the viewing direction light propagating from the light-emissive region to the light-reflective structure.
Preferably the light-reflective structure comprises an electrode for injecting electrical charge into the light-emissive material, most preferably the cathode electrode.
The barrier structure preferably comprises an electrically insulating formation and a light-reflective layer as described above.
The light-emissive material is suitably an organic material and preferably a polymer material. The light-emissive material is preferably a semiconductive and/or conjugated polymer material. Alternatively the light-emissive material could be of other types, for example sublimed small molecule films.
One or more charge-transport layers may be provided between each light-emissive region and one or both of the electrodes.
Each light-emissive region suitably represents a pixel of the display.
In one preferred embodiment the barrier structure consists of or comprises a transparent or translucent dome-shaped or otherwise profiled region (suitably of electrically insulating material) located over the edges of the around the emissive region. This may be defined by the intersection of a metallic cathode and a transparent anode, which reflects, refracts, waveguides or otherwise directs the light emerging from the edge of the emissive pixel redirecting it towards the viewer. The dome-shaped or otherwise profiled region which re-directs the light is suitably located around the periphery of the emissive region in a position such that the light emerging from the edge of the emissive layer is deflected by means such as a mirror or waveguide or both towards the viewer. The emissive region is suitably not covered by this profiled structure, allowing the transparent electrode to be coated, in the window defined by the profiled structure, with one or more organic materials that form the emissive region. The profiled surface of the structure can be coated with a reflective conducting metallic mirror which may be the same layer as that used for a pixel cathode. The profiled structure once formed is preferably thermally stable and will preferably not deform or outgas on subsequent processing during fabrication or during storage or high temperature operation.
One preferred way for the barrier structure to inhibit the propagation of light from one of the light-emissive regions to the other is for said structure to act as a mirror, so that radiation is redirected, either to leave the device in the viewing direction, or else back towards the pixel from which it was emitted (from where it may be directed towards the viewer, by scattering or other processes). There are many methods known within the art for achieving such reflecting properties, and these include the use of metallic mirrors and also the use of dielectric structures in which the reflective properties are achieved by ensuring that propagating optical modes are forbidden (as for example is achieved with dielectric stack mirrors).
In another preferred embodiment the barrier structure consists of or comprises a portion of the emissive layer that extends slightly beyond the emissive region as defined by the overlap or intersection of the cathode and anode and has profiled and reflectively coated edges such that the light being waveguided within the emissive layer can be reflected, refracted, waveguided or otherwise directed into a direction towards the viewer. Thus, in this embodiment, rather than forming the light re-directing structure from a different material the organic electroluminescent material itself is profiled. This is potentially a simpler structure in design and may have advantages in that it may be easier and therefore cheaper to fabricate.
In another preferred embodiment a substrate which has one or more regions coated with a transparent conductor forming isolated pixel anodes is spin coated with a layer of photoresist which is formed into separate islands using standard photolithographic techniques. These separate islands are suitably located above the anodes in areas where the emissive regions will be formed and may have a square or rectangular footprint. The photoresist preferably extends laterally beyond the edges of these emissive regions and preferably has a profile with steep side walls. The substrate may then be heated to allow the photoresist islands to adopt a hemispherical or more generally a rounded profile due to surface tension. The photoresist islands may then be coated with a reflective material such as a metal to form mirrors which are conformant to the profile of the photoresist islands, and are preferably curved. The central portion of the rounded structure may then be removed, e.g. using standard photolithographic techniques. The reflective metallic layer may act as a mask and/or etch stop for the plasma etching of a central window in the photoresist. Thus a window, e.g. of square or rectangular shape, can be formed leaving a curved profiled border around the emissive region. The substrate can then be coated with an organic electroluminescent material and a cathode.
In another preferred embodiment the preferably square or rectangular islands can be formed with their central portion already removed at the first photolithographic stage. This creates a hollow frame-like structure around the emissive region with steep side walls on its inside and outside. After the heating stage the frame-like structure becomes more rounded and forms a bead around the edge of the emissive area with a hemispherical or more generally rounded cross-section profile. This can then be coated with an organic electroluminescent material followed by a reflective metallic coating which can also act as the cathode.
In another preferred embodiment the emissive layer is an organic electroluminescent polymer which is coated directly onto the patterned transparent conductor coated substrate and converted if necessary. Separate square or rectangular islands of photoresist with steep side walls can be formed using standard photolithographic methods. The entire substrate may then be subjected to plasma etching to remove both the polymer and the photoresist to some extent. This suitably results in the polymer layer having an angled side wall profile. A reflective coating which also acts as the cathode can also be deposited on the substrate.
In another preferred embodiment a substrate coated with a patterned transparent conductor is then coated with an insulating material such as an insulating polymer (such as polyimide) which can be patterned and processed using standard photolithography and is thus preferably insoluble in solvents that dissolve photoresist. The substrate can then be coated with photoresist and patterned as in the second embodiment to create a hollow frame-like structure to lie around the emissive region, with steep side walls on the inside and outside. Plasma etching can then remove both the photoresist and the underlying polymer in such a manner as to produce a hollow frame-like structure in the underlying polymer with angled side walls. This structure can then be coated with an organic electroluminescent material and a reflective metallic coating which also acts as the cathode.
Some preferred materials for components (where present) of the display device are as follows:
One of the electrodes (the hole-injecting electrode) preferably has a work function of greater than 4.3 eV. That layer may comprise a metallic oxide such as indium-tin oxide (“ITO”) or tin oxide (“TO”) or a high work function metal such as Au or Pt. The other electrode (the electron-injecting electrode) preferably has a work function less than 3.5 eV. That layer may suitably be made of a metal with a low work function (Ca, Ba, Yb, Sm. Li etc.) or an alloy or multi-layer structure comprising one or more of such metals together optionally with other metals (e.g. Al). At least one of the electrode layers is suitably light transmissive, and preferably transparent, suitably at the frequency of light emission from one or more of the light-emissive regions.
The or each charge transport layer may suitably comprise one or more polymers such as polystyrene sulphonic acid doped polyethylene dioxythiophene (“PEDOT-PSS”), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (“BFA”), polyaniline and PPV.
The or each organic light-emissive material may comprise one or more individual organic materials, suitably polymers, preferably fully or partially conjugated polymers. Suitable materials include one or more of the following in any combination: poly(p-phenylenevinylene) (“PPV”), poly(2-methoxy-5(2′-ethyl)hexyloxyphenylenevinylene) (“MEH-PPV”), one or more PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and related co-polymers, poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (“TFB”), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene)) (“PFM”), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene)) (“PFMO”), poly (2,7-(9,9-di-n-octylfluorene) (“F8”) or (2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (“F8BT”). Alternative materials include small molecule materials such as Alq3.
According to a third aspect of the present invention there is provided a method for forming a display device of any of the types described above.
According to a fourth aspect of the present invention there is provided electronic apparatus (such as a computer display or a portable computer) comprising a display device of any of the types described above.
Any implied orientation of the device is not necessarily related to its orientation during use or manufacture.